MOD 1 - Intro, Patient Dose and IQ

Patient Dose refers to

Absorbed Dose, the dose that is absorbed within the patient and remains in the patient

What are the two interactions that take place for Absorbed Dose to occur

• Photoelectric absorption = x-ray is completely absorbed by an e- and that electron gets ejected and contributes to pt dose

• Compton scatter = photon interacts with an outer shell e- which loses energy and changes direction (it either reaches the detector or stays inside the patient), and the e- gets ejected and absorbed into patients 

Factors that Affect Dose

• mAs

• kVp

• SID

• structures with high atomic number (lung = low, bone = high)

Compton Scatter and IQ

We strive to reduce the scatter striking the image, but overall, compton scatter shouldn’t be completely gone as it creates contrast

kVp and Contrast

• Remember, INVERSE RELATIONSHIP

• Low kVp → INCREASED low-contrast (more attenuation/larger difference in attenuation)

• High kVp → DECREASED low-contrast (less attenuation)

Noise

• few x-rays hit the receptor resulting in “gaps“ in the final image

• image appears mottled or grainy

Scatter

the direction of the x-ray changes within the body, thus hitting the receptor in an area that does not correlate with the anatomical structures

Effects of Increasing kV

• beam penetrability

• pt dose

• scatter

• noise

• CR

• SR

• beam energy increases

• pt dose increases

• scatter increases

• noise decreases

• very high kV = decreased contrast due to scatter

• no change in detail

Effects of Increasing mA

• beam penetrability

• pt dose

• scatter

• noise

• CR

• SR

• no change in beam penetrability

• increased pt dose

• increased scatter, due to increased quantity

• decreased noise

• no change in contrast

• no change in detail

Effects of Increasing SID

• beam penetrability

• pt dose

• scatter

• noise

• CR

• SR

• no change in beam penetrability

• decreased pt dose (intensity is reduced, as per inverse square law)

• decreased scatter

• increased noise

• no change in contrast (less scatter doesn’t necessarily = increased contrast)

• increased detail, due to reduced mag

Effects of Increasing Collimation

• beam penetrability

• pt dose

• scatter

• noise

• CR

• SR

• no change in beam penetrability

• decreased pt dose

• decreased scatter

• possible small increase in noise

• increased contrast, due to less “fogging“

• no change in detail